The current AIDS global pandemic is devastating. Effective HIV vaccines that counteract or prevent AIDS are urgently needed but their development has proven to be extraordinarily difficult. HIV vaccine formulations should have the ability to stimulate long-term memory T cells as well as to induce broadly neutralizing antibodies. This grant proposal focuses on a novel strategy to deliver HIV vaccines. Avirulent Toxoplasma gondii will be evaluated as a live carrier vector for HIV vaccines with the potential to elicit strong long lasting T-cell responses as well as neutralizing antibody responses directed against HIV. This R21 proposal is of an exploratory nature. The protozoan T. gondii has unusual immunogenicity in that it is capable of eliciting a strong Th1 biased cell-mediated immunity against itself as well as nonspecific resistance against other pathogens and tumors. Transgenic T. gondii that have been modified to express epitopes derived from heterologous antigens are capable of eliciting both T-cell dependent host immune responses such as highly effective priming for CD8+ T cell-dependent protective immunity, and host responses that elicit epitope specific antibodies. This parasite is also highly amenable to the types of genetic manipulation required to engineer "safe" strains. In preliminary studies we have developed avirulent uracil auxotrophic mutants of T. gondii. These novel parasite mutants are remarkably avirulent in severely immune deficient mice and immunocompetent mice. Furthermore, a single low-dose of these avirulent parasites inoculated into mice effectively confers a long-term protective immunity to lethal challenge infections. The overall aim of this proposal will be to assess the feasibility of utilizing avirulent mutants of T. gondii to express characterized cytotoxic T lymphocyte (CTL) epitopes, or neutralizing B-cell epitopes, of HIV antigens. The immune responses generated by this novel HIV vaccine delivery strategy will be assessed in murine models.